Pope Francis speaks in the Synod Hall during a conference on Modern Slavery and Climate Change at the Vatican, Tuesday, July 21, 2015. Dozens of environmentally friendly mayors from around the world are meeting at the Vatican this week to bask in the star power of eco-Pope Francis and commit to reducing global warming and helping the urban poor deal with its effects.

New York — When a coalition of mayors from around the world gathered at the Vatican this week to discuss their role in addressing climate change, many of them proclaimed it was up to local leaders like them to take the reins in cutting global emissions.

Later this year, delegations from national governments are preparing to gather for the major United Nations climate conference in Paris. But whereas they will negotiate important new “top down” global carbon-emission protocols, the mayors of major cities – from New York to Johannesburg, South Africa, to Beijing – are taking the “bottom up” approach. And in working to effect change at the local level, they’ve been at the vanguard of reducing emissions throughout the world in the past decade.

It is big cities, after all, that consume two thirds of the world’s energy and then pump out more than 70 percent of the globe’s human-made carbon, according to many studies. What’s more, rising sea levels and more severe weather patterns – some of the possible effects of climate change, many scientist say – could pose a particular threat to cities, since 9 out of 10 major urban areas in the world are near coastal regions.

“The major cities of the world have been working together for the last decade already, and most of them have a serious emissions inventory – sort of an understanding where their emissions come from, a target for a reduction, and a plan for achieving it,” says Mark Watts, executive director of C40, a coalition of 75 of the world’s megacities with populations over 3 million, whose officials meet to discuss the best strategies to cut carbon emissions.

The two-day conference of 60 big-city mayors, which started Tuesday, was part of Pope Francis’ ongoing efforts to promote his recent moral imperative, the controversial encyclical “Laudato Si.” The directive called on global leaders to rethink a world economy based on burning fossil fuels – an economy that exploits the poor and damages the earth, he proclaimed.

The conference also highlighted the environmental strategies that cities around the world have taken, such as changing municipal-level laws and instituting regulations to reduce the globe’s emissions.

“We are called here to take ‘Laudato Si’ and give it life, to systematically address the dangers of climate change with every tool we have,” said New York Mayor Bill de Blasio at the Vatican on Tuesday. “And it is increasingly clear that we, the local leaders of the world, have many tools – more than we may have in fact realized – and that we must use them boldly even as our national governments hesitate.”

Last year, a UN-sponsored “Compact of Mayors” brought together 84 cities that voluntarily agreed to seek dramatic cuts in their carbon emissions and to share ideas and strategies that have proved successful.

That compact is one sign of how local governments may have the most say in global efforts to address climate change, experts say.

“On a rough sort of basis, mayors tend to control most of the serious levers connected to the transport systems, the waste management systems, and tend to have powers over regulating buildings – and therefore the energy consumption in buildings, which is one of the largest sources of greenhouse gas emissions in Western cities, at least,” says Mr. Watts, who attended the meetings at the Vatican this week.

Moreover, mayors tend to be more politically progressive than many national governments.

New York’s efforts, begun during the administration of Mayor Michael Bloomberg, have already decreased emissions by nearly 20 percent since 2005, officials say. And Mayor de Blasio has set a “40 by 30” goal to reduce the emissions of America’s largest urban area by 40 percent by 2030, and a “80 by 50” goal of an 80 percent reduction by 2050, by having more energy-efficient building requirements and a plan for decentralized, cleaner energy sources.

Scandinavian cities remain by far the global leaders in reducing carbon emissions, however. Oslo’s transportation system will run entirely on renewable energy sources by 2020, and in Copenhagen, Denmark, 4 out of 10 workers already travel to their jobs on bicycles.

"Climate negotiators must dare to push boundaries and exclude fossil fuels as an option and reward solutions that are long-term sustainable and renewable," said Stockholm Mayor Karin Wanngård at the Vatican on Tuesday, according to The Associated Press. Most of the Swedish capital’s public transportation network is powered by renewable sources of energy, and Mayor Wanngård is pushing to make the city fossil-fuel-free by 2040.

Meanwhile, Beijing – the city that emits more carbon than any other in the world – has banned new coal, steel, and cement plants as part of its efforts to address its enormous pollution problems. And Shenzhen, China, with a population of 15 million, has become the world leader in putting zero-emission electric vehicles on its streets.

Experts also point to the rapidly developing cities in the Southern Hemisphere as important places to address the problems of pollution and climate change. “These mayors that will have the most control over land-use planning will be the single biggest determinant of whether or not cities can get on a pathway to being successful low-carbon regions,” Watts says.

Johannesburg, a member of the C40, has been one of the most active cities in the Southern Hemisphere seeking to dramatically cut emissions. It has replaced hundreds of taxis and minibuses with energy-efficient buses in its Bus Rapid Transit program, in place since 2009. It has also been retrofitting municipal buildings with energy-efficient lighting, capping landfills to capture methane gas and convert it to electricity, and planting hundreds of thousands of trees.

Though big-city mayors have been working to dramatically cut their carbon footprints for the past decade, the pope’s recent encyclical has energized the movement in a way few have ever seen, many participants said.

“I felt like I was entering a liberated zone,” de Blasio told The New York Times of his experience at the Vatican. The mayor says he’s “spiritual,” but does not identify with an organized religion.

“I go to meetings with mayors practically every week of my life now,” says Watts, “and it was visible in the room how they had been lifted by the pope’s leadership and felt newly confident about sharing more radical leadership themselves.”